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Nuclear DNA of Physarum is made up of 45% repeated (c0t1/2= 0.07–0.7 mol · 1−1· s) and 55% non-repeated base sequences (c0t1/2= 500–1100 mol · 1−1· s) and has a complexity of 130 times the Escherichia coli genome. DNA replicated early in the S phase of the cell cycle, contains few repeated base sequences (below 10%).

Some features of deoxyribonucleic acid (DNA) from 41 Zymomonas strains from Zairese palm wines, Mexican pulque, and English beer and cider were determined. DNA base composition varies within the narrow range of 47.5 to 49.5 mol% guanine plus cytosine. DNA:DNA hybridizations in stringent conditions show high nucleotide sequence similarities of over 76% DNA duplexing (% D), but mostly over 90% D. There is one slightly aberrant strain with less than 32% D. The molecular weight of the Zymomonas genome is 1.5 x 109. Z. mobilis and Z. anaerobia have almost identical genomes. Our data do not support species differentiation.

Summary Drosophila females were treated with caffeine and mated to males that had been exposed to 2000 R X-irradiation; only mature spermatozoa were sampled. The frequencies of radiation-induced dominant lethals, sex-chromosome losses, and II-III translocations in the paternal genome were determined. Treatment of the females with caffeine led to an increase in the frequency of dominant lethals and sex-chromosome loss and to a decrease in the frequency of translocations. If it is assumed that dominant lethals and sex-chromosome loss arise mainly from open, non-restituted breaks, and that translocations are the result of misrepaired breaks, the present findings suggest that caffeine acts by partially inhibiting repair processes acting in the oocytes, processes that are required for the repair or misrepair of chromosome breaks induced in the spermatozoa.

We have isolated plaque-forming λ particles carrying different segments of the temperate phage Mu DNA. These hybrids were obtained by heat induction of dilysogens in which Mu was inserted into the lac genes in λp lac 5 c I857 S 7. The hybrid DNA molecules, derived from three different insertions of Mu, were digested with the specific endonucleases of Hemophilus influenzae , and the resulting fragments were separated by polyacrylamide gel electrophoresis. Comparison of the fragments produced from λp lac 5 c I857 S 7, Mu, and the presumptive hybrids confirmed that a part of the λ genome had been deleted and that portions of Mu DNA had been added. The amount of Mu DNA present did not correspond to the amount of λ DNA absent. Analysis of the fragments obtained from the vegetative Mu DNA indicated that both ends of Mu DNA are variable in length.

With a view to eclucidating chromosome relationships between Lolium perenne (Lp), L. multiflorum (Lm) and Festuca pratensis (Fp), chromosome pairing in different diploid (2n=14), auto-allotriploid (2n=3x=21), trispecific (2n=3x=21), amphidiploid (2n=4x=28) and auto-allohexaploid (2n=6x=42) hybrids between them was analysed. At all these levels of ploidy there was very good chiasmate pairing between the chromosomes of the three species and, on the whole, there was little evidence of preferential pairing of the chromosomes of a particular species in the triploid, tetraploid and hexaploid hybrids. A critical test for this also came from the synaptic ability of the chromosomes of the single genome with those of the duplicated genome in the auto-allotriploids which formed predominantly trivalents with 2, 3 or even 4 chiasmata. Moreover, the homology between the Lp and Lm chromosomes seems strong enough to pass the discrimination limits of the B-chromosomes which do not suppress homoeologous pairing in the Lp LmLm triploid and LpLm diploid hybrids. — The triploids having two genomes of a Lolium species and one of F. pratensis had some male and female fertility which suggested genetic compatibility of the parental chromosomes resulting, presumably, in compensation at the gametic level. Also, the occurrence of comparable chiasma frequencies in the auto-allotriploids and trispecific hybrids showed that they were not markedly affected whether two doses of one genome and one of the other or all the three different genomes from the three species were present. From the trend of chromosome pairing in all these hybrids it is concluded that there is little structural differentiation between the chromosomes of the three species, no effective isolation barrier to gene-flow between them, and that they are closely related phylogenetically, having possibly evolved from a common progenitor. Taxonomic revision of the two Lolium species is suggested.

The pattern of genome segregation to progeny stalked and swarmer cells of Caulobacter crescentus has been determined in a study of the localization of information in developing cells. The genome of stalked cells was labeled with [ 3 H]deoxy-guanosine to mark one of the two DNA strands preferentially. The segregation of this labeled strand after one or more rounds of replication and division in non-radioactive medium was determined by (a) the rate of accumulation of radio-activity during three successive generations of swarmer cells released from labeled stalked cells which were attached to glass plates, and (b) electron microscopy autoradiography of stalked and swarmer cell progeny of labeled stalked cells. The results indicate that most of the DNA of a given age in C. crescentus segre-gates randomly to the two cell types at division, and that the genome probably segregates as a single chromosomal unit.

The composition and metabolism of nonhistone chromosomal proteins associated with the genome of normal and SV40 transformed WI38 human diploid fibroblasts were examined. Evidence is presented which indicates variations in the relative protein content, specific activity and phosphorylation of several defined molecular weight classes of these chromosomal polypeptides. In addition, it is shown that blocking DNA replication with cytosine arabinoside does not inhibit the incorporation of 3 H-leucine into nonhistone chromosomal proteins of SV40 transformed cells. The possibility that the nonhistone chromosomal proteins of human diploid fibroblasts transformed by SV40 virus may be involved in the regulation of viral induced alterations in gene expression is discussed.

Studies with a single-stranded DNA probe complementary to the RNA of mouse-tropic AKR murine leukemia virus indicate that the complete genome of the AKR-type murine leukemia virus is present in the DNA of high- and low-virus-yielding mouse strains, while DNA of non-virus-yielding strains contains only a part of the genome. Furthermore, in those strains where the genome is complete, two populations of virus-specific DNA sequences can be identified (more abundant and less abundant species) according to their rate of association with the probe. Low-virus-yielding mouse strains contain fewer copies of the less abundant species and, consequently, fewer complete viral genomes than do high-virus-yielding strains. Thus, in the ten strains tested, there is a good correlation between completeness of the genome of AKR-type murine leukemia virus in cellular DNA and the capacity of the cells to release infectious AKR-type murine leukemia virus. Moreover, the number of complete viral genomes correlates with the frequency of infectious virus production by virus-positive strains. DNA from wild Mus musculus also contained viral sequences, the sample tested showing reassociation kinetics identical to the non-virus-producing strains.

Previous work with bacteria has shown that a gene is maximally sensitive to mutagenesis by N-methyl-N′-nitro-N-nitrosoguanidine (NG) at the time it is being replicated. NG was used to test for temporal order in the replication of the genome of the unicellular eucaryote, Saccaromyces cerevisiae. Yeast cells growing exponentially were more sensitive to mutagenesis by NG than cells in which DNA synthesis had been inhibited. Further, in a synchronized population of cells, individual genetic markers exhibited maximum sensitivity to mutagenesis at distinct limited intervals within the DNA synthesis period. The peaks of sensitivity are interpreted as reflecting the times of replication of different genes. Since markers for five genes on four different chromosomes showed discrete periods of maximum sensitivity, it is likely that temporal ordering of replication exists for most genes in the yeast genome. These results imply that sites for initiation of DNA replication occur at fairly specific regions along yeast chromosomal DNA molecules, and are activated at predetermined times in the DNA synthesis period.

Polyadenylic acid sequences were detected in bean pod mottle (BPMV) and cowpea mosaic virus (CPMV) RNAs but not in tobacco rattle (TRV) and tobacco mosaic virus (TMV) RNAs or the low molecular weight pathogenic RNA (viroid) from citrus exocortis disease (CEV). A resistant RNA fragment from BPMV-RNA-treated with T 1 and pancreatic ribonuclease migrated as 4–5 S RNA in polyacrylamide gel electrophoresis. The polyadenylic acid sequence appears to be associated with both components of the BPMV genome.

There exists a fairly detailed understanding of the mechanism and regulation of RNA and protein biosynthesis in bacterial cells. This is a direct result of intensive work by a large number of laboratories on a relatively few, especially favorable, systems: bacteriophages T4, T7, and ,\: the RNA phages: and the lac, am, his, and trp operons in Escherichia coli and related bacteria () -4). Progress in understanding the much more complex molecular events that regulate differentiation in eukaryotic cells will, l ikewise, require the sustained and collab­ orative efforts of many research groups working on a few advantageous systems. A wide variety of genetic, biochemical, and morphologic approaches will have to be used, and no one organism or system will be ideal for all these types of studies. The cellular slime mold, Dictyostelium discoideum, offers many advantages for a detailed study of differentiation in a relatively simple system. Dictyostelium is a eukaryotic protist easily cultivated as single cells either axenically or in association with bacteria (5-7). Isolated cells form clones with an efficiency of nearly 100%. The nuclear DNA genome is one fiftieth the complexity of that of mammalian cells (810) and the mode of mRNA biosynthesis is conserv­ ative and involves a much shorter nuclear precursor than in mammalian cells

Chromosome pairing was studied in hexaploid (2n = 42) hybrids of B. erectus (2n = 28) × B. pumpellianus (2n = 56) and B. pumpellianus ssp. dicksonii (2n = 28) × B. pumpellianus. Chromosome pairing in the B. erectus × B. pumpellianus hybrid was complete with predominantly bivalent formation and a low frequency of quadrivalents. The pairing results support the contention that B. pumpellianus is an autoalloploid with an AAAABBBB genome formula. The B. pumpellianus ssp. dicksonii × B. pumpellianus hybrid has an AAABBB genome formula. The presence of quadrivalents, hexavalents and cells containing a total number of bivalents or bivalents plus trivalents in excess of 14 indicated pairing between the A and B genomes. However, a high univalent frequency showed that the A and B genomes were homoeologous rather than homologous. Evidence for genetic control of homoeologous chromosome pairing and homoeologous differentiation between the genomes of the species is discussed.

Satellite phage P4 directs the capsid proteins of its helper phage, P2, to form a head which is only one-third the size of the normal P2 head. The P2 head contains a genome of molecular weight 22 × 10 6 , while the small P4 head contains a genome with a molecular weight of only 7 × 10 6 . We have used in vitro DNA packaging to test whether P2 and P4 phage head sizes are determined by DNA size. The small DNA of satellite phage P4 added to a P2-infected cell extract was packaged primarily into particles containing three copies of the P4 genome. This process occurred with approximately the same efficiency as P2 DNA packaging in the same cell extract. In contrast, the large DNA of P2 was packaged 300-fold less efficiently than the small DNA of P4 in an extract derived from P4-infected, P2-lysogenic cells. These results suggest that DNA size is not sufficient to determine head size. The results are compatible with DNA packaging via the filling of preformed empty capsids.

Replication order to map position discrepancies have been documented in B. subtilis. The discrepancies were found to occur whenever the genome of B. subtilis was replicated under a variety of physiological conditions and in both 168 and W23 strains. The earliest replicating marker involved in these discrepancies was thr versus purB and aroA versus recA. A detailed linkage analysis of the ura to argA region was consistent with its being a continuous linkage group. This led to the conclusion that an origin for new starts at replication exists between recA and aroA.

Several factors which might affect the relative proportions of genomes and antigenomes in Sendai virus have been examined. The relative amount of antigenomes assembled into mature virions was found to be independent of the time of harvest, whereas the amount of cellular RNAs incorporated into virions was found to increase late in infection. Virus strain and host cell did affect the relative amounts of antigenomes in both mature virions and the infected cell, but in all cases more antigenomes were present in the infected cell than would be expected from their function as intermediates in genome replication.

A small-plaque polyoma virus, MPC-1, was isolated from a mouse plasmacytoma. The DNA of this polyoma virus was cleaved with a restriction enzyme from Haemophilus influenzae (Hin d), and the molecular weights of the limit products were analyzed by electrophoresis and electron microscopy. The fragments produced by this enzyme have been ordered by analysis of partial digest products. A physical map of the polyoma virus genome was then constructed.

Cells derived from tumors induced in "nude" mice after injection of cells that were hybrids between mouse peritoneal macrophages and simian virus 40 (SV40)-transformed human cells were found to retain the human chromosome 7 carrying the SV40 genome, and indicate that the presence of human chromosome 7 carrying the SV40 genome is responsible for the expression of the tumorigenic phenotype in the hybrid cells.

Fractionation of DNA of healthy and wilt-infected cotton plants has been carried out according to the reassociation kinetics and the content of GC and 5-methylcytosine in the resulting fractions has been studied. The genome of cotton plant was found to be methylated quite unevenly. The GC rich (GC equals 64.7 mole%) fraction of highly reiterated sequences (Cot equals 0-3.7 times 10- minus 2) has a high content of 5-methylcytosine (5.8 mole%), whereas the methylation degree of the fraction of unique sequences (Cot larger than or equal to 487) is very low (the 5-methylcytosine content is about 0.5 mole%). In plants being infected with wilt, the 5-methylcytosine content in DNA or cotton leaves decreases two-fold; no changes in the structure and molecular population of DNA has been found. The sharp change in the 5-methylcytosine content in DNA of infected plants takes place at the expense of the decrease in the 5-methylcytosine content in fractions of highly reiterated sequences. The methylation degree of unique sequences (structural genes) remains unchanged.

Bacteriophage PM2 supercoiled DNA contains one to three small single-stranded regions that can be detected in the electron microscope after various treatments. The relative positions of these regions were mapped against the unique cleavage site for the restriction endonuclease R · Hap II on PM2 DNA. Any of eight sharply defined regions of the genome may be single-stranded in supercoiled molecules. They are found in all possible combinations of three or less and at approximately the same frequency. A comparison of this map of supercoiled DNA with the alkaline denaturation pattern of nicked circular or linear PM2 DNA showed that these same regions were also the earliest melting regions in non-supercoiled DNA.

A procedure for investigating the possibility of small amounts of partial DNA sequence homology between two defined DNA molecules has been developed and used to test for sequence homology between simian virus 40 and polyoma DNAs. This procedure, which does not necessitate the use of separated viral DNA strands, involves the construction of hybrid DNA molecules containing a simian virus 40 DNA molecule covalently joined to a polyoma DNA molecule, using the sequential action of Eco RI restriction endonuclease and Escherichia coli DNA ligase. Denaturation of such hybrid DNA molecules then makes it possible to examine intra molecularly rather than inter molecularly renatured molecules. Visualization of these intramolecularly renatured “snapback” molecules with duplex regions of homology by electron microscopy reveals a 15% region of weak sequence homology. This region is denatured at about 35 °C below the melting temperature of simian virus 40 DNA and therefore corresponds to about 75% homology. This region was mapped on both the simian virus 40 and polyoma genomes by the use of Hemophilus parainfluenzae II restriction endonuclease cleavage of the simian virus 40 DNA prior to Eco RI cleavage and construction of the hybrid molecule. The 15% region of weak homology maps immediately to the left of the Eco RI restriction endonuclease cleavage site in the simian virus 40 genome and halfway around from the Eco RI restriction endonuclease cleavage site in the polyoma genome.

Summary DNA extracted from a chicken lymphoblastoid cell line (MSB-1) originally derived from a Marek's disease tumour was examined for the presence of the Marek's disease virus genome. DNA cRNA membrane filter hybridization experiments established the presence of 60 to 90 genome equivalents in these cells.

Subcloning of Simian virus 40 (SV40) T antigen-positive mouse-human hybrids, derived from the fusion of mouse cells deficient in thymidine kinase with SV40-transformed Lesch Nyhan fibroblasts, resulted in their segregation into T antigen-positive and negative subclones. Positive correlation between the presence of human chromosome 7 and the expression of SV40 T antigen was established in the subclones examined. These results negate the possibility of a transfer of the SV40 genome to a mouse chromosome.

Sedimentation analysis and intensity fluctuation spectroscopy have been used in conjunction with the Svedberg equation to determine the particle molecular weights of Rous sarcoma virus (Prague strain) and avian myeloblastosis virus (BAI strain). The molecular weights of these two viruses are (294 ± 20) × 10 6 and (256 ± 18) × 10 6 , respectively. Values for the molecular weight of the RNA contained in each particle have been calculated as (5.58 ± 0.5) × 10 6 and (5.88 ± 0.5) × 10 6 . Since the proportion of the viral RNA represented by 4 to 7 S low-molecular-weight material is known, the molecular weight of the 60 to 70 S genomes may be calculated to lie in the range (3.8 ± 0.3 to 4.8 ± 0.4) × 10 6 for both particles. These estimates for the molecular weight of the 60 to 70 S genome are much lower than previous estimates and fall within the range of current estimates of the size of a single 35 S subunit. The implications of this finding are discussed in terms of current theories for the structure of the genome of RNA tumor viruses.

A set of 64 mutants of Saccharomyces cerevisiae that confer sensitivity to X-ray inactivation were analyzed genetically to determine the number of genetic loci involved. The mode of interaction of various combinations of mutants was also determined. A minimum of 17 genes, when mutant, increase X-ray sensitivity of yeast, primarily by eliminating the resistance of budding haploid cells and by removing the shoulder on the survival curves of diploid cells. Eight mutant loci affect principally X-ray sensitivity while the remaining genes also control sensitivity to ultraviolet. Some of the genes when homozygous block sporulation or result in partial or complete sterility. Examination of the survival responses of multiple-mutant strains indicated a minimum of two pathways in the repair of X-ray damage. A number of the mutants have been mapped and these were found to be dispersed over the genome.

The role of rectification of heteroduplex heterozygotes in the formation of recombinant genotypes involving closely linked markers has been examined. Heteroduplex molecules of bacteriophage lambda DNA, heterozygous at several alleles, have been constructed and the genetic composition of phage present in infective centers derived by transfection with such molecules has been determined. Allele loss and concomitant recombinant formation is frequent, and appears to reflect marker specificity as well as specificities imposed by whether or not the transfection recipient is permissive or nonpermissive for DNA duplication of the transfecting genome. The observations support the proposal that many, perhaps most, of the events involving separation of closely linked markers occur by rectification of non-recombinant heterozygotes.

Influenza virus multiplies productively in chick cells and abortively in L cells. The infecting influenza virus RNA genomes are less stable in infected L cells than in infected chick cells. However, transcription of the virus genome in L cells, while reduced in rate, is not decreased in extent. There is no detectable synthesis of virion RNA in L cells, and this is the most likely cause of the abortive infection.

Genetic crosses were made using strains of Saccharomyces cerevisiae which carried cytoplasmically inherited markers conferring resistance to erythromycin, oligomycin and chloramphenicol. The frequency of transmission of these mitochondrial loci to diploid progeny was found to be influenced by the physiological state of the haploid parents, and was not affected by the cis or trans configuration of the three resistance markers. No recombinational polarity was seen in any of the crosses. Growth of a haploid parental strain to stationary phase in a yeast extract peptone medium containing glycerol as a carbon source resulted in a high level of transmission of mitochondrial markers when crossed with a strain grown to stationary phase in the same medium but with glucose as carbon source. When cells were grown under the same conditions as those used in the genetic crosses they were found to contain more mitochondrial DNA relative to nuclear DNA when glycerol was used as a carbon source than when glucose was used. Two criteria were used to determine the amount of mitochondrial DNA present: i) incorporation of radioactive precursors into the different DNA species; and ii) measurement of the mass of DNA from amounts of ultraviolet-absorbing material at the appropriate buoyant densities in isopycnic CsCl gradients. It is proposed that the two to three fold difference in the ratio of mitochondrial DNA to nuclear DNA in stationary phase cells grown in the presence of glycerol compared to glucose reflects an increased number of mitochondrial genomes in derepressed mitochondria. This difference, by a “genome dosage” effect, could account for the variations in the genetic parameter of the frequency of transmission, i.e., strains grown with glycerol as a carbon source contain more mitochondrial genomes than glucose-grown strains and thus will contribute more mitochondrial markers to the zygote.

To monitor the perpetuation of triploid genomes in laboratory-reared progeny of wild-caught females of Poecilia, amounts of DNA-Feulgen staining in scale epithelial nuclei were determined for 58 different fish. Based on estimates of average DNA content, these specimens were readily separated into two groups: (a) diploids, with an average of 1–57 × 10−12 g DNA per cell and (b) triploids, with an average of 2.38 × 10−12 g DNA per cell. These values yield a ratio of 1: 1.5, as would be expected from the presence of an extra chromosome set in putative triploid females. Ploidy status of the latter, based on DNA cytophotometry, was independently confirmed for several specimens by direct cytological examination of chromosome number, by electrophoresis of plasma proteins and by morpho-metric analysis. These findings unequivocally document one case involving the production of triploid offspring from a typical, diploid P. formosa and several instances in which broods of all-female triploid offspring were derived in the laboratory from both wild-caught and laboratory-reared triploid mothers. The relationship and significance of these triploid forms to the breeding complex of P. formosa and P. mexicana in nature are discussed.

Ethidium bromide, an inhibitor of mitochondrial transcription, inhibits phosphate uptake, valine incorporation, and uridine incorporation in fertilized eggs of Strongylocentrotus purpuratus, the purple sea urchin. Phosphate uptake, valine incorporation, and valine influx are also inhibited by this drug in artificially activated nucleate and anucleate egg fragments. Ethidium bromide does not affect the egg respiration and it has no measurable effect on ATP level and labeling. It is concluded that the post-fertilization appearance of the phosphate-carrier system does not require the participation of the nuclear genome but that of the mitochondrial one. It is likely that the same proposal holds for the L-valine-transport system. Some effects of ethidium bromide on development are exposed.

Two common back-pattern morphs are known in seven species of the genus Ranidella in Australia. Inheritance patterns of crosses within and between the two species R. pseudinsignifera and R. insignifera are investigated. The results indicate that the loci involved occupy homologous positions in the two genomes, and have analogous, if not identical, alleles. It is suggested that the polymorphism was present in an ancestral stock species before these two taxa differentiated.

Polyribosomes prepared from both the membrane (bound polyribosomes) and cytoplasmic (free polyribosomes) fractions of tobacco mosaic virus (TMV)-infected tobacco leaves were found to contain small RNAs of several sizes which, by molecular hybridization with denatured double-stranded TMV-RNA, were shown to consist of portions of the TMV-RNA genome. In addition, full-length (30 S) TMV-RNA was found on the free but not the bound polyribosomes. These RNAs were associated with all sizes of polyribosomes as analyzed on sucrose gradients, with the larger species of RNA predominating the regions containing the larger polyribosomes. A heterogeneous population of replicative intermediate molecules was associated with the bound, but not the free polyribosomes. Here, too, the molecules were distributed throughout the polyribosome gradient. The possible functions of the ribosome-associated RNAs as messenger RNAs for viral-coded proteins are discussed.

Recombinants have been isolated from crosses of temperature-sensitive mutants of two adenovirus serotypes whose DNAs differ in their cleavage patterns with restricting endonucleases. Recombinant and parental genomes were dissected with restriction enzymes and the resulting fragments compared. From the results one can align the adenovirus genetic and physical maps.

The level of redundancy of ribosomal genes, and the relationship of this level to nucleolar formation at different stages of embryonic development, have been examined in the Mexican axolotl, Ambystoma mexicanum. Individuals from 4 inbred stocks were examined, as well as descendants from 2 nucleolar variants which, in the heterozygous condition, are distinguished by exceptionally small nucleoli. Ribosomal RNA-DNA hybridization assays show that one of the 4 wild type lines has only about one-third as much ribosomal DNA (rDNA) as the other three. One of the nucleolar variants has the same level of rDNA as the larger wild-type level; the other variant has the same amount as the smaller ribosomal genome line. Both original nucleolar variants arose as F1 progeny of crosses between a large rDNA genome line and the small genome line. Cytological examination of pregastrula stage embryos from wild type and nucleolar variant lines show that the lengths of the nucleolar organizer regions (NOR) and the sizes of nucleoli formed, are directly correlated with the amount of rDNA present at the nucleolar locus. During gastrulation of the nucleolar variants, however, a transition appears to take place and the amount of rDNA ceases to be the determining factor in nucleolar size. After late gastrula, heterozygous progeny resulting from crosses of either large rDNA genome or small rDNA genome wild type individuals with either nucleolar variant line, have a small and a large nucleolus. The factor or factors associated with this apparent lack of competitive ability of the variant NOR, when opposed to a normal NOR, are unknown. It might be suggested that since the chromosomal alterations which produced the nucleolar variants in both cases eliminated the gene determining the dark colour pattern, they could at the same time have eliminated other genetic material.

One hundred and ten amber mutants of coliphage P1 were isolated and localized into groups with respect to the existing genetic map by use of nonpermissive Escherichia coli K-12 strains lysogenic for P1 with deletions. These lysogens contain one of three types of deletion prophages: P1cry and its derivatives, P1dlacs, and P1dpros. Fourteen such lysogens were tested for their ability to rescue the amber mutants which were then assigned to one of nine deletion segments of the P1 genome defined by the termini of the various prophage deletions. The relationship of the nine deletion segments with the published P1 map is described, two new segments having been added. The deletions of the 14 prophages overlapped sufficiently to indicate that the P1 genetic prophage map should be represented in circular form, which is consistent with the fact that P1 is normally a circular plasmid in the prophage state. The distribution of mutants into deletion segments is nonrandom for at least one segment. In addition, the deletion termini of the 14 defective prophages coincided in five out of nine regions separating the nine deletion segments. Various possible explanations are discussed for the nonrandom recurrence of these deletion termini, including the evidence of hot spots of recombination.

E. dasystachys amphiploid. The basic genome formula of the amphiploid is SSHHJJXX, with the SSHH genomes coming from E. canadensis and the JJXX genomes coming from E. dasystachys. Consideration of the morphological, ecological, and reproductive characteristics of all North American species that contain the basic SSHH and JJXX genomes led to the conclusion that A. dasystachyum (Hook.) Scribn., SSHH, and E. triticoides Buckl., JJXX, are the

Summary The simple sequences of Drosophila melanogaster DNA have been investigated after an extensive fractionation of the genome by actinomycin-CsCl and Ag + - Cs 2 SO 4 preparative gradients. In addition to the simple sequence satellites which occur in lengths with molecular weights greater than 10 million daltons, further evidence is presented for the existence of simple sequences of molecular weight less than 10 million daltons which are associated with less rapidly renaturing DNA. This pattern is a property of only a small number of the DNA fractions at 9.5 million daltons. It is not a generalized feature of the main band DNA sequences which are believed to be genetically active. Thus, the simple sequences investigated here represent a class different from that studied by Wu and Bonner [16, 17] which has slower renaturation kinetics and uniform distribution throughout the genetically active genome as 150 base pair long stretches of middle repetitive DNA spacing unique DNA sequences about 750 base pairs in length. The data presented here are consistent with the existence of a special class of non-satellite simple sequence DNA amounting to more than 10 p. cent of the genome and being associated with less redundant DNA sequences at 10 7 daltons, as proposed by Kram et al. [ 6 ].

Chromatin from TLT hepatoma cells, mouse liver cells, and mouse brain cells was fractionated by differential centrifugation into a pellet, enriched with heterochromatin, and a supernatant, enriched with euchromatin. The pellet was found to contain more than twice as much of a particular species of chromatin-associated RNA per milligram chromatin DNA as did the supernatant. This chromatin-associated RNA was also found to be associated with the transcriptionally inactive chromatin of mature avian erythrocytes. Bull sperm, whose genome is known to be completely inactive, was used as the source in the preparation of sperm heads. Bull sperm head RNA appeared to consist of a single, low molecular weight species which migrated on polyacrylamide gels at a rate just slightly slower than the aforementioned chromatin-associated RNA. The results are interpreted as indicating that this chromatin-associated RNA is more prevalently associated with the heterochromatic fraction of chromatin. It is postulated that this chromatin-associated RNA might constitute a structural component of heterochromatin.

Polyacrylamide-slab gel electrophoresis showed that the nature of peroxidases in wheat is a genetic character; different genomes control the synthesis of specific peroxidases. The growing conditions have no influence on isozyme patterns. Within a single species the electrophoretic patterns of peroxidases may differ from one variety to another. Thus, two classes of Triticum aestivum can be distinguished. With Triticum durum the dissimilarity of enzymograms is particularly important.

Summary An attempt was made to hybridize the chicken (Gallus domesticus) male with Japanese quail (Coturnix coturnix japonica) female in order to study the genetic susceptibility of hybrid embryos to avian RNA tumour viruses of subgroups, A, B, D and E. In the hybrids the results supported the prevailing concept that susceptibility is dominant over resistance regardless of the dominant trait contributed by either parent. It was also observed that the Ie gene of the chicken was unable to suppress the ‘quail-coded’ susceptibility to subgroup E virus in the hybrid system, suggesting the lack of penetrance of the Ie gene. Despite the fact that some hybrids were resistant to viruses of subgroups B and D, they were susceptible to subgroup E virus, which was not expected on the basis of the concept that subgroup B-resistant cells cannot be E-susceptible. Also, the hybrids were susceptible to E virus regardless of gs antigen expression and presence of the Ie gene in the genome. This indicates that our earlier suggestion that the Ie gene is another expression of the gs antigen-determining gene is inconsistent.

The size of hnRNA transcripts and the fraction of hnRNA that is converted to mRNA in cell lines of Drosophila melanogaster and Aedes albopictus are compared. Both insects belong to the order Diptera, but Aedes has a 5–6 fold larger genome than does Drosophila. The Aedes line produces significantly (2-2.5 fold) larger hnRNA than does the Drosophila line, even though the two cell lines grow under similar conditions and produce mRNA of the same size and sequence complexity. These data suggest that within a given taxonomic order, the size of hnRNA increases with increasing genome size. The fraction of hnRNA converted to mRNA [cytoplasmic poly(A) + RNA] has been measured for the two cell types by comparing initial rates of labeling of hnRNA with initial rates of appearance of labeled mRNA in the cytoplasm. While 20% of the Drosophila hnRNA is converted to mRNA, only 3.3% of the Aedes hnNRA is converted to mrRNA. The poly(A) content of the hnRNA from the two species is also different; Drosophila hnRNA has approximately three times as much poly(A) as does Aedes hnRNA. The data show—at least for these two species—that the average amount of hnRNA transcribed relative to the amount of mRNA formed increases as genome size increases. The data are consistent with the interpretation that more DNA is transcribed into hnRNA in Aedes, the organism with the larger genome, than in Drosophila.

The role of polyploidy in the evolution of the sweet potato. I. batatas (2n=6x=90), became more clear in 1971 when wild species with 30, 60, and 90 chromosomes were discovered. These species, I. leucantha (2x), I. littoralis (4x) and I. trifida (6x), are the progenitors of the sweet potato (6x), in an autopolyploid series with doubling of the I. leucantha B genome. In the present study the hypothesis of the origin of the sweet potato was confirmed by comparative studies on some plant characters, sexual compatibility, and the behavior of artificial hexaploids produced from I. leucantha (2x) and I. littoralis (4x). Since induced hexaploid I. leucantha exhibits weak growth, a few multivalents are formed at meiosis in the sweet potato, and there are differences in morphological and physiological characteristics between the artificial hexaploids and I. batatas, raw autoploidy seems unlikely, but a balanced or diploidised autoploidy could have been achieved by genic and chromosomal changes in the course of evolution. However, there is still sufficient homology so that meiotic pairing occurs usually between regular partners but also between homoeologous chromosomes in a certain situation.

Skin grafts transplanted from B 10.HTT donors onto (A.TL X B 10)F 1 recipients are rapidly rejected despite the fact that the B10.HTT and A.TL strains should be carrying the same H-2 chromosomes and that both the donor and the recipient contain the B10 genome. The rejection is accompanied by a production of cytotoxic antibodies against antigens controlled by the Ir region of the H-2 complex. These unexpected findings are interpreted as evidence for a third histocompatibility locus in the H~ complex, H-2I, located in the Ir region close to H-2K. The B 10.HTT and A.TL strains are postulated to differ at this hypothetical locus, and the difference between the two strains is explained as resulting from a crossing over between the H-2 tl and H-2 s chromosomes in the early history of the B10.HTT strain. The H-2 genotypes of the B10.HTT and A.TL strains are assumed to be H-2KsIrs/kSskH-2D d and H-2KsIrkSskH-2D d, respectively. Thus, the H-2 chromosomes of the two strains differ only in a portion of the Ir region, including the H-2I locus. The B 10.HTT(H-2 tt) and B10.S(7R)(H-2 th) strains differ in a relatively minor histocompatibility locus, possibly residing in the Tla region outside of the H-2 complex.

An attempt was made to transfer the murine sarcoma virus genome from cryptically transformed HT-1 cells to hamster embryo cells via isolated chromosomes (chromosome immigration). Chromosome immigration did not result in any transformation of recipient embryo cells. However, there was transfer of a rescuable sarcoma virus genome. Evidence indicates that the transfer requires the intact chromosome structure. It was not possible to identify one or any chromosome associated with the rescuable sarcoma genome.

A tryptophan tRNA derived from the host cell serves as primer for the initiation of DNA synthesis by the reverse transcript ase of avian sarcoma virus (ASV). We have characterized the binding of primer to the viral genome by studying the reassociation of low molecular weight RNA's with high molecular weight subunits of ASV-RNA. Primer can be annealed to a limited number (two to four) of sites on the viral genome to reconstitute a template fully active with reverse transcriptase. The annealing is specific and accurate; none of the other 4 S RNA's found in ASV will reassociate with viral genome under any of the conditions tested, and the reconstructed complex denatures with the same T m as the native complex between primer and viral genome. These results further substantiate our previous conclusion that the primer in ASV is a single species of tRNA, although the normal host cell may contain other 4 S RNA's which could also serve as primer if they were included in the viral genome during assembly. Chicken ribosomal 5 S RNA also reanneals to the ASV genome, presumably by binding to the sites originally occupied by 5 S RNA in viral 70 S RNA.

Cleavage of simian virus 40 DNA I with the restriction endonuclease from Hemophilus aegyptius yields 16 fragments of approximately 18 to 1520 nucleotides. These fragments have been ordered and precisely aligned with the fragments composing the H. influenzae and H. parainfluenzae endonuclease cleavage maps of SV40 † by a combination of two methods: (1) hybridization of radioactively labeled transcripts of Hae fragments to denatured Hin fragments; (2) reciprocal digestion of Hin fragments with H. aegyptius endonuclease and Hae fragments with the endonucleases from H. influenzae and H. parainfluenzae , followed by size analysis of the resultant products. In addition, nucleic acid hybridizations have been performed between the transcripts of certain Hae fragments and DNA from the adenovirus-SV40 hybrids, Ad2 + ND 1 , Ad2 + ND 4 and E46 + , which permit more precise localization of the distal termini of the SV40 segments within these hybrids on the map of the SV40 genome.

The deoxyribonucleic acid (DNA) of the mycoplasma Thermoplasma acidophilum has a base composition of 46 mol% guanine plus cytosine (G+C). Depurination can occur to the DNA in the culture conditions, giving rise to a product of lower buoyant density. This effect probably accounts for earlier descriptions of a much lower G+C content in the DNA. The size of the T. acidophilum genome, as measured by DNA renaturation, was 30% the size of the Escherichia coli genome.

DNAs from three grasshopper species and three locust species (Family Acrididae, Order Orthoptera) have been characterised by a variety of biochemical techniques as part of a general investigation of their molecular evolution. Base compositions were inferred from thermal denaturation profiles and analytical CsCl and Cs2SO4 pcynography. Repeated sequence content of the genomes was studied by renaturation kinetics incorporating absorption measurements. Relative genome size was measured by microdensitometry of nuclear preparations.